Northwestern University scientists have found out with a thermoelectric material that is the best in the world at converting waste heat to electricity.
This is very good news once you realize nearly two-thirds of energy input is lost as waste heat.
The material could signify a paradigm shift. The inefficiency of current thermoelectric materials has limited their commercial use. Now, with a very environmentally stable material that is expected to convert 15 to 20 percent of waste heat to useful electricity, thermoelectrics could see more widespread adoption by industry.
Possible areas of application include the automobile industry (much of gasoline's potential energy goes out a vehicle's tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places were large combustion engines operate continuously (such as in large ships and tankers).
Waste heat temperatures in these areas can range from 400 to 600 degrees Celsius (750 to 1,100 degrees Fahrenheit), the sweet spot for thermoelectrics use.
The new material, based on the common semiconductor lead telluride, is the most efficient thermoelectric material known. It exhibits a thermoelectric figure of merit (so-called "ZT") of 2.2, the highest reported to date. Chemists, physicists, material scientists and mechanical engineers at Northwestern and Michigan State University collaborated to develop the material.
"Our system is the top-performing thermoelectric system at any temperature. The material can convert heat to electricity at the highest possible efficiency. At this level, there are realistic prospects for recovering high-temperature waste heat and turning it into useful energy," said Mercouri G. Kanatzidis, who led the research and is a senior author of the paper.
The performance of the new material is nearly 30 percent more efficient than its predecessor. The researchers achieved this by scattering a wider spectrum of phonons, across all wavelengths, which is important in reducing thermal conductivity.
"Every time a phonon is scattered the thermal conductivity gets lower, which is what we want for increased efficiency," Kanatzidis said.
A phonon is a quantum of vibrational energy, and each has a different wavelength. When heat flows through a material, a spectrum of phonons needs to be scattered at different wavelengths (short, intermediate and long).
In this work, the researchers show that all length scales can be optimized for maximum phonon scattering with minor change in electrical conductivity.
"We combined three techniques to scatter short, medium and long wavelengths all together in one material, and they all work simultaneously," Kanatzidis said.
"We are the first to scatter all three at once and at the widest spectrum known. We call this a panoscopic approach that goes beyond nanostructuring."
In particular, the researchers improved the long-wavelength scattering of phonons by controlling and tailoring the mesoscale architecture of the nanostructured thermoelectric materials. This resulted in the world record of a ZT of 2.2.
The successful approach of integrated all-length-scale scattering of phonons is applicable to all bulk thermoelectric materials, the researchers said.
The study will be published in the journal Nature.